While grocery shopping I go armed with two very special and important tools, both of which help keep me on budget.
Tools in my pocket:
I'm sure you have heard before how important it is to follow a lost while shopping, but there are two things that make if effective. First is that you checked to make sure you actually NEED what is on the list. I can't tell you how many times I brought groceries home and while putting them away found what I had just purchased already in my fridge! Second, sticking to it. Stores actually market products to you by putting them in the aisles or prominent places so you will be tempted to buy them and they are usually NOT the cheapest variety of each product either.
I am constantly using the calculator app on my phone while I am shopping. I use it to figure out unit prices-like I explained in a previous post. The other thing I use it for is to keep a running total of my purchases. Usually when I do that it keeps me from buying impluse items or over buying in general. If i know I have $X for that shopping trip and my total is getting there quickly and I still have a lot more items on my list, I know I will need to cut back on some things. It also keepa me from looking like a fool at the checkout counter when I don't have enough money to buy my groceries! :)